Digital Video - September 2008 - (Page 32)

DV 101 BY JAY HOLBEN FUN WITH FILTRATION TO GEL OR NOT TO GEL, THAT IS THE QUESTION! I n previous DV101 columns and blogs, I’ve talked about colorcorrection filters for lighting. Here, we’re going to delve a bit deeper into color-correction filters. Since this is a more advanced discussion, I’ll assume that you, dear reader, have a basic understanding of color temperatures in general. (If not, take a look at my DV101 Blog on color temperature at DV.com.) Color-correction filters for lighting are often called “gels,” as they used to be made from natural animal-product gelatin, but they are now most often a polycarbonate plastic, which lasts longer, is more robust and has a higher melting point. There are four major types of color-correction gel: CTB (blue), CTO (orange), plus green (green) and minus green (magenta). In addition, you’ll find CTS (straw) from most manufacturers. CTS has the same color-correction properties as CTO but provides a slightly yellower tone that some users find more pleasing. There are several primary manufacturers of color filters: Lee, Rosco and GAM. All companies make a fantastic product, and the decision to go with one over the other most often comes down to personal choice and regional availability. There are, however, differences between the companies and between the specific correction filters, as I’ll discuss in a bit. Color correction can be somewhat confusing for many people. Not only is there Blue and Orange and Green and Magenta to deal with, but there are various strengths to each! All the primary correction filters come in 1⁄8, 1⁄4, 1⁄2, 3⁄4 and Full versions — each variation providing greater color shift in their correction. So, how and when do you break these out? Wouldn’t you just always use Full CTO or Full CTB? Well, no. The first thing is that realworld daylight is almost never 5600°K and tungsten is almost never 3200°K. In fact, you’ll often see individuals refer to daylight as 5500°K or 5600°K or even 6500°K — because there is no real set temperature and each manufacturer or individual will use their own base number. These numbers are theoretical standards that rarely happen in the real world. Natural daylight can range from 3000°K to 20,000°K depending on where you are on the globe, the cloud cover, the time of day, the season, the pollution and so forth. Tungsten lights can range from 2000°K to 3600°K depending on the manufacturer, age of bulb, reflector, lens and so forth. Even putting two tungsten lights from the same manufacturer next to each other, you’re more likely to get two different color temperatures out of them than you are to get them to match! Because there are so many variables, color-correction filters dv september 2008 IT SHOULD BE NO SURPRISE TO LEARN THAT NOT ALL FILTERS ARE CREATED EQUAL. come in variable strengths to help compensate for the discrepancies. Understanding how these strengths integrate into realworld applications requires the introduction of a little math. Most people understand that a Full CTO filter will convert daylight (5600°K) to tungsten (3200°K), a color temperature shift of 2400°K. However, what if the daylight, at the time of your shoot, is actually 7000°K? Does that mean that Full CTO will make it 4600°K (70002400)? Actually, no. To get a better understanding of how much a filter will affect the color, you need to get into mireds. A mired is a microreciprocal degree. It’s derived by dividing 1,000,000 by the Kelvin temperature. So 5600°K has a mired of 179 (1,000,000 / 5600 = 179). If you look at the manufacturer swatchbooks for lighting filters, you’ll see that each filter has a mired shift factor. Lee’s Full CTO has a mired shift of +159. 179 + 159 is 338. 1,000,000 / 338 = 2,958, which rounds to 3,000°K. So, Lee’s Full CTO actually corrects 5600°K to 3000°K, not 3200°K. If, in the example above, you actually had 7000°K instead of 5600°K, 7000 has a mired value of 143. 143 + 159 is 302. 302 mired is 3300°K. So Lee Full CTO on a daylight source of 7000°K would give you a color temperature of 3300°K. This system of mathematical calculation is important to understand so that you can compensate properly. It can also be used inversely. If you have a color temperature meter and you www.dv.com 32 http://www.dv.com http://www.dv.com

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Digital Video - September 2008 Contents Discproducer PP-100 Frameforge 3D Studio 2 First Cuts Expodisc Bird's-Eye View Instant Expert Inside Outsource Red in Combat Tools & Technology My Studio DV101 Production Diary

Digital Video - September 2008

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